Three major types of chemical bonds exist: covalent, ionic, and hydrogen. The form of bond that is established is determined by a specific arrangement between the electrons. Ionic bonds are formed when electrons are exchanged between two atoms and the resulting bond is relatively weak. For example, salt is held together by ionic bonds between sodium (Na ) and chloride (Cl-) ions. Covalent bonds occur when electrons are shared between atoms; this form of bond is strongest and is found in both energy-rich molecules and molecules essential to life. For example, hydrogen and oxygen molecules in water are held together by covalent bonds. Hydrogen bonds are temporary, but they are important because they are crucial to the shape of a particular protein and have the ability to be rapidly formed and reformed, as in the case of muscle contraction. The following chart summarizes the three types of chemical bonds and their characteristics: